I used 300 x 300 pixel art which is the size they recommend, and have tried jpg and png formats. Whether you need an album cover ASAP because you have a song release date rapidly approaching, or you just want to know what your cover art will look like before you buy, pre-made album covers helps you do just this. The other tags seem fine, and I can update those and save, its just the 'Album cover' one that is giving the problem. When I click on the 'pencil' icon in the album cover placeholder, it comes up with the choose image panel fine, and I can choose an image, but when I click on the image, I just get the busy symbol (circle rotating), eventually this time out, and the image is not updated (or changed). When I go into the Groove Music app and right click on a music file and choose 'Edit Info' and am taken to the Edit Album Info panel, I find I can change all the info apart from the Album cover.

Its not a security question, so I hope its ok posting this issue to see if people know an answer or at least cn tell me if its a generic Windows 10 O/S issue, and not just my computer.

Groove music album art